Earth has kept a detailed diary of its history, and each layer of rock is a page in that
diary.

The further back, however, the more those rocks have metamorphosed and eroded, which means the
earliest chapters in our planet’s history are missing.

One of those chapters is called the Late Heavy Bombardment. A realignment of the planets,
especially massive Jupiter, sent asteroids careening through the solar system 4.2 billion to 3.8
billion years ago.

Many of the craters on the moon were formed at that time. Earth would have suffered even more
impacts — it’s a bigger target, and its larger gravity would have pulled in more missiles — but
little evidence of those impacts has survived.

One of the oldest impacts on Earth is the Vredefort Dome in South Africa, which formed 2 billion
years ago. Two recent papers in the journal
Geology discuss the crater.

The Vredefort Dome is 100 miles in diameter with deformation extending 12 miles into the
crust.

Immediately after impact, the crater would have filled with a lake of molten rock that
eventually cooled, hardened and was later stripped away by erosion. None of that has been found,
until now.

The rock, with the euphonious name of foliated gabbronorite, occurs as a network of thin sheets
in the granite of the central uplift. It contains crystals of the mineral zircon that formed at
temperatures of 1,450 to 1,700 degrees.

The authors suggest that looking for similar rocks elsewhere might lead to the discovery of
older impacts, including those from the Late Heavy Bombardment.

The other article reported on two cores from Karelia, Russia, acquired by the Fennoscandian
Arctic Russia-Drilling Early Earth Project. The cores contained spherules, or droplets of molten
rock thrown into the atmosphere by an impact.

The spherules themselves can’t be dated, but they occur between rocks that can. Those rocks are
2.05 billion and 1.98 billion years old, which are consistent with the date of the Vredefort
impact. If so, they are the oldest spherules found from a known crater.

The size of the spherules indicates that the impact was less than 1,550 miles away, a useful
piece of information for reconstructing the positions of the continents long before the assembly of
Pangea.

The history of early impacts on Earth is important for determining when life might have started
on our planet. It might have occurred soon after Earth formed 4.6 billion years ago, although
scientists aren’t sure life could have survived the Late Heavy Bombardment.

Fossils that are 3.5 billion years old have been found, and chemical hints (including low carbon
isotope ratios) suggest life was present even earlier, perhaps 3.8 billion years ago.

If life started so soon after Earth’s formation, it might mean that life is a natural event in
the evolution of a planet, such as the appearance of quartz.

If that is the case, with more than 1,700 exoplanets that we know of, the universe might be
brimming with life.

Dale Gnidovec is curator of the Orton Geological Museum at Ohio State University.